Question

Define \(B O D\) and \(C O D\), and explain why their values for the same water sample can differ slightly. Explain why natural waters can have a high \(\mathrm{BOD}\).

Short answer

BOD measures biodegradable organic matter, while COD measures all organics. Natural waters can have high BOD due to organic runoff and decaying matter.

Step-by-step solution

Defining BOD

Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen required by aerobic microorganisms to break down organic matter in water over a specified period, usually 5 days at 20°C. It is an important indication of the organic pollution level in water.

Defining COD

Chemical Oxygen Demand (COD) is the total quantity of oxygen required to oxidize all organic and inorganic matter present in the water. COD is determined by a chemical oxidation process, typically using a strong oxidizing agent, such as potassium dichromate.

BOD vs. COD in the Same Water Sample

BOD and COD values can differ slightly because BOD only measures the oxygen demand of biodegradable organic material, while COD measures both biodegradable and non-biodegradable (recalcitrant) organic substances. COD generally gives a higher value than BOD due to the inclusion of non-biodegradable organics.

Why Natural Waters Can Have a High BOD

Natural waters can have a high BOD due to the presence of high concentrations of organic materials like leaves, decaying plants, animal waste, and other biodegradable organic matter. Additionally, agricultural runoff or effluents containing organic compounds can contribute to increased BOD levels.

Key concepts

Chemical Oxygen Demand (COD)

Chemical Oxygen Demand (COD) is a crucial metric used to understand the pollution potential of water. It specifically measures the total amount of oxygen required to chemically oxidize organic and inorganic substances in a water sample. By chemically oxidizing, we mean the process that involves adding a chemical oxidant, such as potassium dichromate, to a water sample to convert these substances into their oxidized forms. Unlike BOD, which relies on the biological breakdown of organic matter, COD uses a purely chemical method and can measure both biodegradable and non-biodegradable organics.
This makes COD a more comprehensive and quicker way to assess water quality compared to BOD. While BOD focuses only on the biodegradable portion, COD considers all oxidizable substances, providing a higher and more inclusive oxygen demand value. This is especially important in industrial and wastewater treatment applications, where understanding the total potential of organic pollution is crucial.

• Comprehensive as it includes both biodegradable and non-biodegradable materials.
• Uses chemical oxidation, a quick method for assessment.

Understanding COD is fundamental for environmental monitoring and helps guide efforts in water treatment processes, ensuring safe and clean water supplies.

Aerobic Microorganisms

Aerobic microorganisms play a vital role in the natural process of breaking down organic matter present in water bodies. These organisms require oxygen to perform their metabolic activities, which include decomposing biodegradable substances. In doing so, they utilize the available dissolved oxygen in water, which reflects in the Biochemical Oxygen Demand (BOD) value.
In environments like sewage treatment plants or natural water bodies, these microorganisms are essential as they help reduce organic pollution by transforming it into simpler compounds, often resulting in cleaner water. However, if the organic pollution load is too high, it can deplete oxygen levels significantly, challenging the survival of these aerobic bacteria and negatively impacting the aquatic environment.

• Aerobic organisms need oxygen to break down organic materials.
• They are crucial in reducing pollution by natural biodegradation.

This concept is significant in understanding why managing organic pollutants is critical in maintaining the health of aquatic ecosystems and ensuring that the BOD levels are balanced to support diverse aquatic life.

Organic Pollution

Organic pollution refers to the presence of materials in water that stem from living organisms and are rich in carbon, such as sewage, agricultural runoff, and decaying plant material. When these substances enter water bodies, they provide a food source for microorganisms, leading to increased microbial activity.
This increased activity necessitates more oxygen as microorganisms work to decompose these organic substances, often resulting in heightened Biochemical Oxygen Demand (BOD) levels. High BOD levels indicate significant organic pollution, which can lead to oxygen depletion in water, harming aquatic life, and leading to conditions like fish kills or algae blooms.

• Defined by the presence of carbon-rich substances from living sources.
• Causes increased microbial activity and oxygen demand.

By understanding the dynamics of organic pollution, effective strategies can be developed to mitigate its adverse effects, ensuring the sustainability of aquatic environments.

Biodegradable Organic Matter

Biodegradable organic matter refers to organic materials in water that can be decomposed by natural biological processes, usually involving microorganisms. This type of matter includes substances like food waste, paper, and agricultural wastes, which are typically broken down by aerobic microorganisms through a process that requires oxygen.
The presence of biodegradable organic matter is essential in determining the Biochemical Oxygen Demand (BOD) of a water sample. As aerobic microorganisms decompose these substances, they consume dissolved oxygen, indicating the potential organic pollution level of a water body. The focus on biodegradable organic matter in BOD assessments offers insights into how effectively biological organisms can clean the water.

• Includes materials like food waste and plant matter, decomposable by microorganisms.
• Affects BOD as it determines the oxygen used by microorganisms for decomposition.

Understanding biodegradable organic matter is crucial in water quality management as it helps assess the capacity of a water body to self-purify and maintain an ecological balance.